Evidence of a structural quantum critical point in (CaxSr1-x)3Rh4Sn13 from a lattice dynamics study

Approaching a quantum critical point (QCP) has been an effective route to stabilize superconductivity. While the role of magnetic QCPs has been extensively discussed, similar exploration of a structural QCP is scarce due to the lack of suitable systems with a continuous structural transition that can be conveniently tuned to 0 K. Using inelastic x-ray scattering, we examine the phonon spectrum of the nonmagnetic quasiskutterudite (CaxSr1-x)(3)Rh4Sn13, which represents a precious system to explore the interplay between structural instabilities and superconductivity by tuning the Ca concentration x. We unambiguously detect the softening of phonon modes around the M point on cooling towards the structural transition. Intriguingly, at x = 0.85, the soft mode energy squared at the M point extrapolates to zero at (-5.7 +/- 7.7) K, providing the first compelling microscopic evidence of a structural QCP in (CaxSr1-x)(3)Rh4Sn13. The enhanced phonon density of states at low energy provides the essential ingredient for realizing strong-coupling superconductivity near the structural QCP.